Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for radio frequency (RF) applications. However, these, more familiar, semiconductor materials may not be well suited for higher power because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
In light of the difficulties presented by Si and GaAs, interest in high power and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths than gallium arsenide (GaN) and GaN typically has better electron transport properties than silicon.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped (“unintentionally doped”), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2. Unlike electrons in conventional bulk-doped devices, electrons in 2DEG may have higher mobilities due to reduced ion impurity scattering.
This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Pat. No. 6,316,793, to Sheppard et al., which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
It is standard practice to screen RF power devices, for example, HEMTs, with high temperature, reverse bias (HTRB) tests as a part of the qualification procedures. The VGS used during HTRB tests may be at least as negative as −2*(2−VGQ) or −2*(2−VT), whichever is more negative, where VGQ is the quiescent VGS for the target application and VT is the threshold voltage of the device. Both VGQ and VT are typically negative. In this case, both VGQ and VT are referenced to the maximum possible gate voltage, which is typically from about 1.0 to about 3.0 Volts. The VDS used during HTRB may be about 2*VDQ, where VDQ is the quiescent VDS for the target application. In particular, the power device may be subjected to the maximum reverse bias voltage that the device may instantaneously reach in a real application, with the temperature raised to normal operating temperature, for example, 140° C. Typically, tests are run for approximately 1000 hours.